This invention relates to hybrid couplers which introduce an inherent guadrature relationship, or 90.degree. phase shift, to electromagnetic waves coupled between two waveguides and, more particularly, to a coupling device including phase shifters in each waveguide to compensate for the 90.degree. phase shift.
Hybrid couplers are widely used in microwave circuits for coupling a portion of the electromagnetic energy in one waveguide to another waveguide. In some cases, the coupling ratio is one-half so as to produce an equal split of the power among the two waveguides. In other cases, a smaller amount of the power such as one-quarter or one-tenth of the power may be coupled from one waveguide to the second waveguide. In a common form of coupler, known as a hybrid coupler, the two waveguides are brought contiguous to each other and in parallel relationship so as to share a common wall. An aperture in the common wall provides for the coupling of the electromagnetic energy.
A problem arises in that the hybrid coupler introduces a 90.degree. phase shift to an electromagnetic wave coupled from the first waveguide to the second waveguide. In many power splitting situations, such as in the corporate feed for a phased-array antenna, it is desirable to maintain equality of phase between the electromagnetic waves in the two waveguides. Due to the presence of the 90.degree. phase shift, or quadrature relatioship, between the waves in the two waveguides, it becomes necessary to introduce a phase correction to the radiant energy of the two waveguides.
One solution to this problem has been the introduction of a phase shifter into the first waveguide, downstream from the coupling aperture, to equalize the phase shifts in the two waveguides. The 90.degree. phase shift in the second waveguide is a phase lag and, accordingly, the phase shifter comprises a series of capacitive elements disposed along the first waveguide.
The foregoing solution to the problem suffers from the disadvantage that a capacitive phase shifter of sufficient length to introduce the required 90.degree. phase shift is unduly long and heavy for use in a microwave network for feeding an antenna. Such microwave networks typically have numerous waveguide branches. Thus, the introduction of additional length and weight to the components of the network causes a significant enlargement of the entire network which renders the network less favorable for installation in mobile applications such as with antennas carried by satellites. Yet a further disadvantage of the foregoing solution is the fact that such phase shifters have proven to be highly dependent on frequency with the resultant phase shift being frequency dispersive. Accurate compensation for the quadrature relationship has only been obtained for a single value or narrow range of frequency. Accordingly, the foregoing problem of the unwanted phase shift has not been resolved in a manner which allows for the reduction in size and weight of the microwave components.